With this build i prefer to play a semi agressive game, which relies on Warding a lot. By this i mean i dont use any wards in the game for vision, i only use trinket to give me options if i'm getting ganked, or if im going for some play. This build in itself has a lot of magic penetration, not percentage but pure penetration. I usually see people following their build which builds void staff pretty agressively, but you should press tab before doing that, and you can see, the enemy doesnt buy so much magic resist usually, in these cases, buying a void staff is a waste. With katarina you can outplay your "counters" so you have to be conscious all the time and usually jumping in for an all in doesnt pay off, so this is a little slower build but more effective. As you're a katarina player, you should take many opportunities to roam, bot or top. If you get a good roam early, that might just be enough to win a game as stupid as it sounds. So, good luck and have fun, I hope you the best.